How Blobb calculates your calories
Every number Blobb shows you comes from the equations below. Where we have made a judgement call rather than used a published value, it says so.
These are estimates, and the honest error bars are wide. Even a well-chosen equation lands within about 150–200 kcal of your real resting rate for two people in three, and within 300–400 kcal for 19 in 20. Treat your target as a starting point to adjust from, not a measurement.
None of this is medical or nutritional advice. See section 2 of the Terms.
If you are 18 or older
We estimate your resting metabolic rate — the energy your body uses at rest — with the Mifflin-St Jeor equation:
Men: 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
Women: 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161
The Academy of Nutrition and Dietetics has recommended this as the standard equation for healthy adults since 2005, and it is what MyFitnessPal, Cronometer and Yazio use. It predicts within 10% of measured values for roughly 70–82% of people, and is less accurate at higher body weights.
If you are under 18
Mifflin-St Jeor was built from adults aged 19 to 78, and it has no term for growth — so it underestimates what a teenager needs, by roughly 7% for girls and 12% for boys before any deficit is applied. For under-18s we use the Institute of Medicine Dietary Reference Intake equations for ages 9–18 instead:
Boys: 88.5 − 61.9 × age + PA × (26.7 × weight(kg) + 903 × height(m)) + 25
Girls: 135.3 − 30.8 × age + PA × (10 × weight(kg) + 934 × height(m)) + 25
These give total daily energy directly, so unlike the adult path they are not multiplied by an activity factor afterwards.
Activity
Four bands. For adults the multiplier is applied to your resting rate; for under-18s the coefficient goes inside the equation above.
Sedentary — adult ×1.2 · PA 1.00
Lightly active — adult ×1.375 · PA 1.13 (boys) / 1.16 (girls)
Active — adult ×1.55 · PA 1.26 (boys) / 1.31 (girls)
Very active — adult ×1.725 · PA 1.42 (boys) / 1.56 (girls)
Why we show each band as a number of steps
Activity levels are hard to judge, and picking too high is the most common reason a calorie target ends up too generous. So we show what each band is worth in energy, expressed as a number of steps:
steps = (energy at this band − energy at sedentary) ÷ energy per step
It is an equivalence, not a step target. If you lift weights and never walk, you can still pick a higher band honestly — the steps are just a way to picture the energy. We derive the figure from the calorie difference rather than borrowing published step-count categories, because those describe how far people walk, not how much energy a multiplier grants. The two differ by roughly 50%.
Turning that into a target
For a weight goal we add or subtract a daily amount based on the rate you chose, using 7,700 kcal per kilogram of body mass:
daily change = weekly rate (kg) × 7,700 ÷ 7
This is the familiar "3,500 calories per pound" rule. It assumes your energy needs stay constant as you lose weight, which they do not — metabolic rate falls as body mass does. Over the weeks-long goals Blobb sets, the error is small. Over a year, this rule would promise roughly twice the loss you would actually see, which is why Blobb does not project that far.
Floors
Blobb will not set a target below a floor, no matter what rate you pick.
- Adults: 1,200 kcal, or 1,000 kcal on the Accelerated plan. These are conventional figures rather than values from a specific study.
- Under 18: your resting energy — what your body burns lying still — from the Schofield equations (boys 10–18: 17.5 × weight + 651; girls 10–18: 12.2 × weight + 746). A growing body should never be pointed below that. Because it is a high floor relative to a teenager's daily needs, it also holds an under-18 to a gentler rate of loss than an adult would get, without refusing outright.
Sex at birth
The sex term in both equations stands in for fat-free mass, which is the largest single predictor of resting metabolism. That is why Blobb asks for sex at birth rather than gender.
There is no third option because there is no third equation. No published equation or guideline estimates resting metabolism without using sex assigned at birth, and averaging the two has never been validated. An app that offers a third choice is either mapping it to one of the two behind the scenes or inventing a number — Blobb used to invent one, in three different places, which is part of why this page exists.
If you are transgender, non-binary or gender non-conforming
If you are not taking gender-affirming hormones, select your sex assigned at birth. It will most closely match your metabolic rate.
If you are, it depends on how long. In the first few months, sex assigned at birth is still the closer estimate. After longer on hormones your metabolism may be closer to your gender identity, as muscle and fat mass shift. Your doctor is better placed than we are to say where you are in that.
Either way, you can set your calorie goal directly in Plan settings and ignore our estimate entirely. That is the honest answer for anyone these two options do not describe — including anyone who has had their metabolism actually measured.
What we do not do
- We do not measure your metabolism. Only indirect calorimetry does that, and it needs a lab.
- We do not use body-fat percentage. Equations that do — Katch-McArdle, Cunningham — are more accurate when you have a real measurement, and worse when you have a guess.
- We do not adjust your target based on what you log. Your estimate changes when your weight, age, activity level or goal changes, and not otherwise.
Sources
- Mifflin MD, St Jeor ST, et al. “A new predictive equation for resting energy expenditure in healthy individuals.” Am J Clin Nutr, 1990.
- Academy of Nutrition and Dietetics, Evidence Analysis Library — determination of resting metabolic rate in adults.
- Schofield WN. “Predicting basal metabolic rate, new standards and review of previous work.” Hum Nutr Clin Nutr, 1985 — adopted by WHO/FAO/UNU.
- Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fibre, Fat, Fatty Acids, Cholesterol, Protein and Amino Acids, 2005 — Estimated Energy Requirement equations for ages 9–18.
- Hall KD, et al., on the limits of the static 3,500 kcal rule. Int J Obes, 2013.